On Sat, Jan 4, 2025 at 1:46 PM Baoquan He <bhe@xxxxxxxxxx> wrote: > > On 12/31/24 at 01:46am, Kairui Song wrote: > > From: Kairui Song <kasong@xxxxxxxxxxx> > > > > The flag SWP_SCANNING was used as an indicator of whether a device > > is being scanned for allocation, and prevents swapoff. Combined with > > SWP_WRITEOK, they work as a set of barriers for a clean swapoff: > > > > 1. Swapoff clears SWP_WRITEOK, allocation requests will see > > ~SWP_WRITEOK and abort as it's serialized by si->lock. > > 2. Swapoff unuses all allocated entries. > > 3. Swapoff waits for SWP_SCANNING flag to be cleared, so ongoing > > allocations will stop, preventing UAF. > > 4. Now swapoff can free everything safely. > > > > This will make the allocation path have a hard dependency on > > si->lock. Allocation always have to acquire si->lock first for > > setting SWP_SCANNING and checking SWP_WRITEOK. > > > > This commit removes this flag, and just uses the existing per-CPU > > refcount instead to prevent UAF in step 3, which serves well for > > such usage without dependency on si->lock, and scales very well too. > > Just hold a reference during the whole scan and allocation process. > > Swapoff will kill and wait for the counter. > > > > And for preventing any allocation from happening after step 1 so the > > unuse in step 2 can ensure all slots are free, swapoff will acquire > > the ci->lock of each cluster one by one to ensure all allocations > > see ~SWP_WRITEOK and abort. > > Changing to use si->users is great, while wondering why we need acquire = > each ci->lock now. After setup 1, we have cleared SWP_WRITEOK, and take > the si off swap_avail_heads list. No matter what, we just need wait for > p->comm's completion and continue, why bothering to loop for the > ci->lock acquiring? > Hi Baoquan, Waiting for p->comm's completion must be done after unuse is called (unuse will need to take the si->users refcound, so it can't be dead yet), but unuse must be called after no one will allocate any new entry. That is guaranteed by the loop ci->lock acquiring.